glFrustum describes a perspective matrix that
produces a perspective projection. The current matrix (see glMatrixMode)
is multiplied by this matrix and the result replaces the current matrix,
as if glMultMatrix were called with the following matrix as its argument:

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Typically, the matrix mode is GL_PROJECTION, and (left, bottom,
-near_val) and (right, top, -near_val) specify the points on the near clipping
plane that are mapped to the lower left and upper right corners of the
window, assuming that the eye is located at (0, 0, 0). -far_val specifies
the location of the far clipping plane. Both near_val and far_val must be
positive.

Use glPushMatrix and glPopMatrix to save and restore the current
matrix stack.

Depth buffer precision is affected by the values specified
for near_val and far_val. The greater the ratio of far_val to near_val is,
the less effective the depth buffer will be at distinguishing between surfaces
that are near each other. If

$r ~=~ "far_val" over "near_val"$

roughly
$log sub 2 (r)$ bits of depth buffer precision are lost. Because $r$ approaches
infinity as near_val approaches 0, near_val must never be set to 0.